Self-cutting fast construction screw

ABSTRACT

A self-cutting fast construction screw for the fastening of fiber plates to a metal substructure carries on the screw shaft at least one turn of a thread. The screw core passes into a conical drill point. On the conical drill point two cutting edges are arranged in a symmetrical layout, said cutting edges cutting to a diameter located between the diameter of the screw core and the diameter of the outer thread tip. Upon impact on the metal substructure, the two cutting edges break off.

BACKGROUND AND OBJECTS OF THE INVENTION

The invention concerns a self-cutting fast construction screw to fastenfiber plates to a metal substructure, with a screw shaft carrying atleast one turn of a thread and passing over into a conical drillingpoint, and with two cutting edges located near the end of the shaft andcapable of breaking off upon impact on the metal substructure.

To fasten mineral fiber plates and the like to a metal substructureconsisting for example of sheet metal supports, customarily self-tappingconstruction screws are used (DE-OS No. 25 39 365), in which the core ofthe screw carries a two-turn (or double helix) thread, one turn of whichextends to the conical point of the screw. After the penetration of thescrew point into the metal substructure, this thread begins to cut athread.

The screw threads in the mineral fiber plate to be fastened are alsoformed by the point of the screw, whereby the material is not removedfrom the bore, but merely displaced laterally. However, for highstrength fiber plates solidly pressed of a fibrous material the use ofthese known construction screws causes problems, as the material to bedisplaced offers great resistance to the the screw, so that a relativelyhigh axial force must be applied to make the penetration of the fastconstruction screw into the fiber plate possible. The threads providedfor fastening in the metal substructure must pass through the fibermaterial, so that a finished thread is shaped into the compressed fibermaterial preventing the establishment of a solid joint between the steelsubstructure and the fiber material, since the fiber material is alreadycompressed laterally, an appreciable frictional force is generated sothat a very high torque must be applied to rotate the construction screwinto the material.

In the case of construction screws of the aforementioned type, intendedfor the fastening of wood panels to a steel substructure, it is known toprovide two opposing cutting edges on the lower end of the screw shaft,both of which cut into the wood to the outer diameter of the thread. Thecutting edges produce a bore hole in the wood, through which the threadsmay pass without appreciable friction with the wood. The edges break offupon their impact on the metal. For the mounting of fiber plates made ofhigh strength fibrous materials, however, these known constructionscrews are not suitable, because here again the drill point would haveto produce the entire bore hole to the core diameter of the screw shaft.

It is therefore the object of the invention to develop a fastconstruction screw of the aforedescribed type that would be suitable foruse with fiber plates made of high strength fibrous materials.

SUMMARY OF THE INVENTION

This object is attained according to the invention wherein the twocutting edges are arranged on the conical drill point and are cutting toa diameter located between the diameter of the thread core and the outerdiameter of the threads, and that the threading terminates on the screwshaft.

As the two cutting edges are located on the tapering drill point, thedrill point must displace the fiber material to a diameter that isappreciably smaller than the diameter of the screw core only. The drillpoint participates in the cutting and displacement of the fiber materialonly with its front section protruding past the cutting edge, so thatthe resistance offered to the drill point is slight. In the rest of thediameter sections on the outside, the two cutting edges effect thecutting of the fiber material.

As the planned fracture line of the two cutting edges is located on thetapering drill point and thus on a diameter that is smaller than thediameter of the screw shaft, the torque required to break off thecutting edges is relatively low; the two cutting edges therefore breakoff immediately upon their impact on the metal of the substructure. Ithas been found that the front section of the drill point is narrow andsharp enough to penetrate into the metal under the effect of the axialforce applied during the screwing process alone, so that the cuttingnotches costumarily provided heretofore in the circumferential surfaceof the drill point are no longer necessary, but they may be additionallyprovided.

When the drill point penetrates deeper into the metal, it is necessaryfor the rear zone of the drill point to displace a larger volume of themetal. However, the fracture surfaces of the two broken cutting edgeslocated in that zone, act as cutting notches or edges, so that in thatzone the material is displaced not only by the applied axial force to,but at least in part by a cutting or squeezing drilling process.

As there are no cutting edges on the shaft of the screw, the thread orthreads (in case of a multiple turn thread) commencing here are notaffected by the breaking of a cutting edge or the remaining fracturesurface.

In contrast to fast construction screws on which at least one turn ofthe thread extends onto the conical drill point, in the case of the fastconstruction screw according to the invention the thread cutting processbegins only after the bore in the metal substructure has been producedto the thread shaft diameter.

In a further development of the concept of the invention, the twocutting edges are mounted at the same axial height on the conical drillpoint. This results in the fact that the torque required for thebreaking off of the cutting edges is actuated simultaneously by both ofthe cutting edges. However, this torque is relatively low, because theonset of the engagement by the cutting edges of the metal substructureand thus the initiation of the fracture are located on a relativelysmall diameter. The symmetrical layout of the two cutting edges requiresa symmetrical action of forces, whereby the fast construction screw isbeing centered during its penetration into the metal.

Appropriately therefore the two cutting edges have the same radialdimensions so that even during the cutting of the fiber material thereare symmetrical forces present, thereby centering the screw.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is made more apparent below by an example of embodimentshown in the drawing. In the drawing,

FIG. 1 shows a self-tapping fast construction screw in a lateralelevation and

FIG. 2 a partial view in the direction of the arrow II in FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The fast construction screw shown is made of steel and has a screw head1 in the shape of a truncated cone, a cylindrical screw shaft 2 and aconical drill point 3. The screw head 1 is provided with cross recessesfor receiving a tool, and, carries on its conical circumferentialsurface a plurality of ribs 4 to facilitate the penetration of the screwhead 1 into the fiber plate to be fastened, for example a mineral fiberplate.

On the screw shaft 2, two thread turns 5, 6 of different height, arelocated; they extend to the lower end of the screw shaft 2. The screw isin the form of a two-turn or double helix screw.

On the conical drill point 3, designed with an apex angle ofapproximately 30°, two cutting edges 7 are mounted at a distance fromthe point of the screw, on opposing generating lines, said cutting edgesbeing in the form of small, flat tabs with a curved, preferablyapproximately semicircular outer edge. The two cutting edges 7 arelocated at the same axial height on the drill point 3 and have the sameradial dimensions.

With their curved outer edges the cutting edges 7 cut the material ofthe mineral fiber plate to a diameter located between the diameter ofthe thread core (diameter of the screw shaft 2) and the thread pointdiameter of the higher turn 5. In this manner a bore is produced in themineral fiber plate into which the shaft of the screw may penetratewithout hindrance. The higher thread turn 5 penetrates slightly into thebore wall of the mineral fiber plate and guides the fast constructionscrew. The lower thread turn 6 touches the bore wall only slightly ornot at all.

The two cutting edges 7 are active during the penetration of the mineralfiber plate to be fastened and are cutting a relatively large proportionof the volume of the bore hole to be formed. Only a smaller volume ofthe bore is produced by the front section of the drill point 3, i.e.,the section located in front of the two cutting edges 7.

When the front section of the drill point 3 has penetrated into themetal substructure, the two cutting edges impact the metal and breakoff. The fracture edges or surfaces produced act during the subsequentpenetration of the drill point into the metal as cutting edges ornotches.

As indicated in FIG. 1 by a broken line, on the front section of thedrill point 3 two opposing cutting notches 8 may be provided, whichfacilitate the penetration of the drill point 3 into the metal in thecase of greater sheet metal thicknesses. The cutting notches 8 are alsoadvantageous in the mounting of asbestos fiber plates or the like.Generally, however, such cutting notches are not necessary.

What is claimed is:
 1. A self-cutting construction screw for fastening afiber plate to a metal substructure, said screw comprising a head, ascrew shaft extending from said head and carrying at least one helicalthread which terminates on said shaft, a conical drill point extendingfrom an end of said shaft situated opposite said head, said drill pointconverging in a direction away from said head, and two outwardlyprojecting frangible cutting edges mounted on said conical drill pointfor cutting through the fiber plate and breaking off upon impact withthe metal substructure, said cutting edges sized to cut to a diameterlocated between the diameter of the thread core and the outer diameterof the thread.
 2. A screw according to claim 1, wherein said head, shaftand drill point define a longitudinal axis, said cutting edges situatedat the same longitudinal height on said drill point.
 3. A screwaccording to claim 1, wherein said cutting edges project radiallyoutwardly from said drill point by equal distances.
 4. A screw accordingto claim 1 including cutting notches provided in said conical drillpoint.
 5. A screw according to claim 1, wherein said at least onehelical thread comprises a pair of helical threads of different outerdiameters.